The present invention relates in general to DC motors, and more particularly to control of a polyphase, brushless DC motor in a disk drive assembly used to move the disks past a read/write head.
Conventional brushless DC motors are highly efficient and possess characteristics that make them suitable for a number of applications. In particular, brushless DC motors are favored as the moving force for rotating the disk surfaces of the disks of hard disk drive systems past a read/write head.
Polyphase, brushless DC motors require application of currents to stator windings in a sequential order to produce torque inducing flux for moving a rotor. DC currents are alternately switched about the stator windings to create various current paths that produce magnetic flux orientations in a synchronized fashion. The resultant magnetic flux produces a torque on the permanent magnet rotor that causes rotational movement. In order to insure that current is applied to the most appropriate current path through the stator windings (the appropriate motor phase) to most efficiently produce the maximum torque, various schemes have been used to provide positional information about the rotor, specifically, the location of the north and south magnetic poles of the permanent magnet rotor.
One such sensing scheme utilizes Hall effect sensors. Other systems have been developed using optical sensors. One problem with these sensing schemes is that the components used in the sensing scheme are often the part of the motor that is most prone to failure, thereby significantly affecting the overall reliability of the apparatus. Additionally, incorporating the components of the sensing scheme in the motor structure itself increases the size, cost, complexity and power consumption of the motor itself.
Another system for initiating and controlling the movement of brushless DC motors in the context of a Winchester type hard disk drive is described in U.S. Pat. No. 4,876,491, issued on Oct. 24, 1989 to Squires, et al. The Squires, et al. patent describes a circuit and method for controlling a brushless DC motor which involves determining the position of the rotor by applying a high frequency current to each of the phases of the motor and then comparing the voltage produced across each phase by that current. The method and circuit described by Squires, et al., requires the use of a microprocessor which increases the cost of the unit and also places some restrictions on the responsiveness of the system because of the delays caused by the use of microprocessor.
Therefore, a significant improvement in brushless DC motor design can be achieved if positional information can be obtained without incorporating extra apparatus in the motor structure itself and without the use of a microprocessor.